Maximal aerobic capacity at several ambient concentrations of carbon monoxide at several altitudes.

In order to assess the combined effects of altitude and acute carbon monoxide exposure, 11 male and 12 female subjects, nonsmokers in good health, were given incremental (two minutes at each workload) maximal aerobic capacity tests at four levels of ambient carbon monoxide (0, 50, 100, and 150 parts per million) at four altitudes (55, 1,524, 2,134, and 3,048 m). Five male and four female subjects completed all 16 experiments. The remaining subjects completed either eight or 12 experiments; at least eight male and eight female subjects were tested at each combination of carbon monoxide and altitude. Test conditions were double-blind. Subjects initially were screened with a medical history questionnaire, a 12-lead electrocardiogram, pulmonary function tests, anthropometric and body fat measurements, blood volume determinations, and a maximal aerobic capacity test. Each subject, after attaining the required altitude and ambient carbon monoxide level, performed the maximal aerobic capacity test (maximum VO2) meeting required conditions to assure that a maximal level was attained. Blood samples were drawn prior to the aerobic capacity test; at workloads of 50 watts, 100 watts, 150 watts, and maximum; at the fifth minute of recovery; and prior to repressurization to sea level. Blood was analyzed for hemoglobin, hematocrit, plasma proteins, lactates, and carboxyhemoglobin. Carbon-monoxide-carboxyhemoglobin uptake rates were derived from the submaximal workloads. Maximum VO2 was similar at 55 m and 1,524 m, and decreased from the 55-m value by 4 percent at 2,134 m and by 8 percent at 3,048 m. Despite increases in carboxyhemoglobin, no additional significant decreases in maximal aerobic capacity were observed. With increasing carbon monoxide, a decrease in maximum VO2 independent of altitude was observed. Carboxyhemoglobin concentrations at maximum VO2 were highest at 55 m and lowest at 3,048 m. Carboxyhemoglobin concentrations were lower in female subjects than in male subjects. Immediately prior to and at maximal workloads, carbon monoxide shifted into extravascular spaces and returned to the vascular space within five minutes after exercise stopped. We demonstrated that altitude hypoxia and carbon monoxide hypoxia act independently on the parameters of the maximal aerobic capacity test. We also demonstrated a decrease in the carbon monoxide concentration to carboxyhemoglobin as altitude increased, which can be attributed to the decrease in driving pressure of carbon monoxide at altitude.